Methods for forming a silicon oxide film through oxidation of a silicon surface may be generally classified into a thermal oxidation process using an oxidation furnace or a rapid thermal process (RTP) apparatus, and a plasma oxidation process using a plasma process apparatus.
As an example of a thermal oxidation process, in a wet oxidation process through an oxidation furnace, a silicon substrate is heated to a temperature of more than 800° C. (degree Celsius) and exposed to an oxidation atmosphere by using a water vapor generator (WVG), thereby oxidizing a surface of the silicon substrate to form a silicon oxide film thereon. Such a thermal oxidation process is considered as a method for forming a silicon oxide film of good quality. The thermal oxidation process, however, requires a treatment at a high temperature of more than 800° C. such that thermal budgets are increased to cause deformation of a silicon wafer due to thermal stress.
On the other hand, with respect to a plasma oxidation process, WO 2004/008519 proposes a method of forming a silicon oxide film by performing a plasma oxidation process in which a silicon surface reacts with a microwave-excited plasma formed at a process pressure of 133.3 Pa by using a process gas containing argon gas and oxygen gas and having a flow rate ratio of oxygen of about 1%.
According to the method disclosed in WO 2004/008519, the plasma oxidation process is performed at a relatively low process temperature of about 400° C., which can avoid the problems such as an increase in thermal budgets and deformation of a substrate that may occur in a thermal oxidation process.
Also, WO 2004/008519 discloses that a plasma is formed under conditions of low pressure and low oxygen density in which a flow rate ratio of oxygen is about 1% and a process pressure is 133.3 Pa. The plasma oxidation process is performed on a silicon surface under conditions such as low pressure and low oxygen density so that a higher oxidation rate may be obtained. Defects of Si—O combinations, however, are introduced into a silicon oxide film that is obtained through a plasma oxidation process under the conditions of low pressure and low density. Such defects cause an initial withstanding voltage to be poor to thereby reduce the yield of devices. This problem may be improved by performing a plasma oxidation process under conditions at a relatively high process pressure (e.g., 400 Pa) and a relatively high oxygen density (e.g., 20%). However, as process pressure and oxygen density increase, the oxidation rate is dropped to thereby reduce throughput. In addition, the increase in process pressure and oxygen density deteriorates a flatness of a surface of a silicon oxide film or a Si/SiO2 interface so as to cause degradation in durability as an insulating film.
Therefore, Japanese Laid-Open Patent Publication No. 2008-91409 proposes a method of forming a silicon oxide film with superior insulating properties while not damaging the advantages of a plasma oxidation process under conditions of low pressure and low oxygen density. This method includes a first oxidation treatment process for forming a silicon oxide film using a plasma that is formed under first treatment conditions in which a ratio of oxygen in a process gas is 1% or less and the pressure ranges from 0.133 to 133 Pa, and a second oxidation treatment process for forming a silicon oxide film using a plasma that is formed under second treatment conditions in which a ratio of oxygen in a process gas is 20% or more and the pressure ranges from 400 to 1333 Pa.
To follow a recent miniaturization of a semiconductor device, it is required in a gate insulating film of, for example, a transistor, a flash memory device or the like to have a film quality whose insulating property does not deteriorate even when stress is repeatedly applied and with a high insulation durability so as to sufficiently restrain a leak current from occurring. Also, in recent years, from a perspective of improving total throughput by way of increasing sheets of semiconductor wafers to be processed, there is a need to upgrade an oxidation rate so as to form a silicon oxide film with a predetermined film thickness in a short period of time. With respect to the two requirements having a different direction as above, it was difficult to satisfy both of them at the same time with the conventional plasma oxidation process methods.
For example, Japanese Laid-Open Patent Publication No. 2008-91409 discloses a method of forming a silicon oxidation film through a two-step plasma oxidation process comprising a condition of low pressure and oxygen density and another condition of relatively higher pressure and oxygen density than those in the former condition. This method intends to obtain a fine silicon oxide film with less defects by maintaining a high oxidation rate as well as the flatness of a surface of the silicon oxide film and a Si/SiO2 interface through the low pressure and oxygen density condition while suppressing defects in the silicon oxide film due to the low pressure and oxygen density conditions. However, such technique essentially employs the two-step plasma oxidation process such that there still remains room for improvement in terms of throughput.